Article pubs.acs.org/JPCC
Aggregation and Electrolyte Composition Effects on the Efficiency of Dye-Sensitized Solar Cells. A Case of a Near-Infrared Absorbing Dye for Tandem Cells Marcin Ziółek,*,† Jerzy Karolczak,†,‡ Maciej Zalas,§ Yan Hao,∥ Haining Tian,⊥ and Abderrazzak Douhal*,# †
Quantum Electronics Laboratory, Faculty of Physics, Adam Mickiewicz University, Umultowska 85, 61-614 Poznan, Poland Center for Ultrafast Laser Spectroscopy, Adam Mickiewicz University, Umultowska 85, 61-614 Poznan, Poland § Faculty of Chemistry, Adam Mickiewicz University, Umultowska 89b, 61-614 Poznan, Poland ∥ State Key Laboratory of Fine Chemicals, DUT-KTH Joint Education and Research Center on Molecular Devices, Dalian University of Technology (DUT), Dalian 116024, P. R. China ⊥ School of Chemical Science and Engineering, Department of Chemistry, KTH Royal Institute of Technology, 10044 Stockholm, Sweden # Departamento de Química Física, Facultad de Ciencias Ambientales y Bioquímica, and Inamol, Universidad de Castilla-La Mancha, Avda. Carlos III, S.N., 45071 Toledo, Spain ‡
S Supporting Information *
ABSTRACT: Time-resolved laser spectroscopy studies of complete solar cells sensitized with a near-infrared absorbing dye (HY103) and filled with different electrolytes are applied to explain their macroscopic parameters (efficiency and short-circuit current). Particular attention is paid to the effect of coadsorbent, size of cations in electrolyte (lithium vs guanidine ones), and addition of tert-butylpyridine. A complete deactivation scheme in the cell is revealed, and the rates of electron injection and all other processes are explored. For the most efficient electrolyte, the electron injection rate constants are 0.21 ps−1 from monomers and 0.07 ps−1 from H-aggregates. Moreover, two important and novel findings are revealed: energy transfer from the excited state of monomers to H-aggregates (with rate constants from 0.04 to 0.25 ps−1) and the decrease of internal conversion rate in HY103 attached to the nanoparticles (0.01 ps−1) with respect to that of free dye in solution (0.06 ps−1). Thus, our study gives more clues to better understand the photobehavior of dye-sensitized solar cells.
1. INTRODUCTION
knowledge about the real rates of the charge separation processes, and the number of such studies is still lacking. One of the ways to improve the efficiency of the solar cells is the use of tandem cells, proved by the success of multijunction semiconductor solar cells.14 Such tandem concepts have been also tried for DSSCs, and one of the most efficient devices among them is that based on TH305 and HY103 (Figure 1A) dyes, giving the total sunlight conversion efficiency of 11.5%.15 The redox potential of the excited state of TH305 is sufficiently negative (−1.28 V vs NHE16), so it is possible to energetically shift the TiO2 conduction band edge to increase the opencircuit voltage of the solar cell. Recently, we have studied the complete solar cells sensitized with this dye by time-resolved laser spectroscopy techniques.17 We have found a nice correlation of electron injection rate and open-circuit voltage of the device, and we confirmed that electron injection
Dye-sensitized solar cells (DSSCs) belong to one of the most promising emerging photovoltaic technologies.1−3 The best solar to electricity power conversion efficiencies of DSSCs are about 12%.4,5 To increase their performance, a new way of decreasing the photovoltage losses in DSSC devices is required.6 Such losses occur mainly due to large driving forces that are necessary to obtain high quantum yields of partial charge separation processes: electron injection from the excited dye to the conduction band of TiO2 nanoparticles and dye regeneration by redox couple in electrolyte (usually I3−/I−). These processes occur in the fast and ultrafast time regime (from femto- to microseconds), and time-resolved laser spectroscopy techniques have to be applied for their investigation.7 Moreover, recent results have showed that the rates of these charge separation processes are quite different in complete, operating DSSCs than in isolated systems studied before.8−13 Therefore, new time-resolved laser spectroscopy studies of complete solar cells are important to have a good © 2013 American Chemical Society
Received: October 11, 2013 Revised: December 10, 2013 Published: December 10, 2013 194
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For the opaque cells, the procedure used for preparation of the titania electrodes was similar to those described elsewhere21 and was as follows: 3 mL of titanium tetraisopropoxide (Aldrich) was added to 13.5 mL of ethylene glycol (Chempur) magnetically stirred at 60 °C. The mixture, after addition of 12.6 g of citric acid monohydrate (POCh), was heated under stirring at 90 °C, until clear. The obtained transparent solution was mixed with 5.6 g of P25 TiO2 (Degussa) by grinding in agate mortar for 1 h. The viscous titania paste obtained was spread on fluorine-doped tin oxide (FTO) conductive glass substrate (Solaronix) using a “doctor blading” technique and sintered in air at 450 °C for 1 h. To prepare working electrodes for DSSCs, titania electrodes were immersed in 3 × 10−5 M solution of HY103 dye with or without CDCA (5 × 10−5 M) in DCM at 5 °C in the dark overnight. After dye adsorption, the electrodes were washed with DCM and dried in a hot air stream. Platinum film coated FTO was used as a counter electrode. The typical cell was assembled using a 25 μm thick, hot-melted, ionomeric foil (Solaronix) as a sealant and a spacer between the electrodes, and the electrolyte was injected within two holes predrilled in the counter electrode. The final sealing was realized with the use of hot melted sealant and a microscope cover slide. The typical active area of the obtained DSSC was approximately 0.125 cm2. For each configuration three cells were prepared, and the shown results are for the best one. For the transparent cells, Ti-Nanoxide HT paste (Solaronix SA) was employed to make the TiO2 electrodes. For the reference electrodes, the alumina paste was obtained by mixing 1 g of Al2O3 nanoparticles (d < 50 nm, Sigma-Aldrich) dispersed in a solution containing: 100 mL of ethanol, 1 mL of distilled water, and 1 mL of acetic acid. With the aim to obtain a homogeneous solution, the suspension was kept at ultrasonic bath during 24 h. Then, 1.6 g of ethyl cellulose and 10 mL of αterpineol were added into the solution (under stirring conditions and temperature of 60 °C). To get the desired viscosity, the excess of ethanol was removed with a rotary evaporator. A conductive glass sheet (FTO) of 2.5 cm × 2.5 cm was first cleaned in a detergent with water, then with distilled water, and finally with ethanol using an ultrasonic bath. A layer of paste was coated on the FTO electrode using the doctor blade technique between two parallel adhesive Scotch tapes. Finally, the film on the glass was gradually heated to 450 °C and kept at that temperature for 1 h to fabricate the TiO2 layer. Then, the conductive glass with the titania or alumina film was immersed in a 3 × 10−5 M solution of HY103 dye with or without CDCA (5 × 10−5 M) in DCM (the same as for opaque cells) and kept at room temperature for 1 h to ensure optimum sensitizer uptake (absorbance of around 1.0 in the maximum of the HY103 visible absorption band for titania sample). The platinized counter electrode was obtained by spreading a Ptbased solution (Platisol T, Solaronix) on FTO glass by heating at 450 °C. To make a complete solar cell, the counter electrode was assembled with thermal adhesive film (25 μm Surlyn, Meltronix, Solaronix) that acts as a separator and sealing element. The different electrolytes (as well as bulk ACN for the reference samples) were introduced in the complete cells with a vacuum backfilling through a hole drilled in the counter electrodes, which was later sealed by a piece of Surlyn and a microscope coverslip. The typical active area of the obtained DSSC was approximately 1.5 cm2. Five different electrolytes were used. The commercial iodidebased redox electrolyte was the one from Solaronix (Iodolyte
Figure 1. (A) Molecular structure of HY103. (B) Current−voltage curves for different opaque solar cells.
quantum yield can maintain a value close to 100% when the conduction band edge is shifted by electrolyte composition. In this contribution, we focus our attention on the second dye of the tandem deviceHY103. On the contrary to TH305, it was found that for good enough efficiency of HY103sensitized solar cells a special electrolyte composition with proper amount of lithium iodide and without pyridine derivatives had to be used to move the conduction band edge of TiO2 toward more positive potential.15,18 This is because the redox potential of the excited state of HY103 is estimated to be much more positive (−0.87 V vs NHE18) than those of TH305 and other common dyes for DSSCs. The HY103 dye is also interesting from another point of view as the novel structure in which the anchoring group is separated from the acceptor groups of the dye (Figure 1A), allowing us to tune the HOMO−LUMO level in an easier way by modifying the structure of the acceptor units.18 Here we study complete DSSCs made of HY103 with the use of conventional current−voltage studies, stationary absorption and emission measurements, and modern time-resolved laser spectroscopy techniques. We compare the effect of adding lithium iodide, 4-tert-butylpyridine, and guanidine thiocyanate into iodide-based liquid electrolytes. The first two additives are the most commonly used in DSSC electrolytes,2 while guanidine thiocyanate was found to reduce dark current of the solar cells facilitating the self-assembly of a compact dye monolayer on nanoparticles.19,20 We also study the dye aggregation effect. The macroscopic parameters of the studied solar cells are correlated with the measured rates of the charge separation and competing processes. Very interesting results are revealed, important for a better understanding of DSSC operation and its optimization.
2. EXPERIMENTAL SECTION HY103 dye (Figure 1A) was synthesized as previously described.18 Acetonitrile (ACN, spectroscopic grade >99.5%, Aldrich) and dichloromethane (DCM, spectroscopic grade >99.5%, Aldrich) were used as received. Chenodeoxycholic acid (CDCA, Sigma-Aldrich) was used as coadsorbent. Two kinds of solar cells were prepared: opaque and transparent ones, both made using slightly different methods. 195
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AN-50), and it contains the I−/I3− redox couple ([I2] = 50 mM), ionic liquid, lithium salt, and pyridine derivative in ACN solution. The other four electrolytes (EL1−EL4) were homemade and were the following: EL1, 0.6 M 1-propyl-3methyl-imidazolium iodide (Sigma-Aldrich), 0.03 M iodine (POCh), 0.1 M guanidine thiocyanate (Fluka), and 0.5 M 4tert-butylpiridine (TBP, Sigma-Aldrich) in acetonitrile (Merck); EL2, the same as EL1 but without TBP; EL3, 0.6 M 1-propyl-3methyl-imidazolium iodide (Sigma-Aldrich), 0.06 M lithium iodide (Sigma-Aldrich), 0.1 M tetrabutylammonium iodide (Sigma-Aldrich), and 0.02 M iodine in acetonitile:valeronitrile 85:15; EL4: the same as EL3 but with higher concentration of lithium iodide (0.5 M). For the broadband femtosecond−picosecond transient absorption measurements, the Ti:Sapphire laser system22 and noncollinear optical parametric amplifier (NOPA) were used. The pump pulses were at 580 or 645 nm, and the IRF (pump− probe cross correlation function) was 150−200 fs (fwhm). The pump pulse energy was 270 nJ, corresponding to energy density of about 1 mJ/cm2. The probe beam was the white light continuum generated in a 1 cm cell of water. The transient absorption measurements were performed in the spectral range of 420−720 nm and the time range of 0−500 ps. The samples (solar cells) were not moving; however, no degradation was observed during experiment, and the time constant values were averaged from three experiments made in different parts of the cells. The data were analyzed using a global fit program that performs a convolution of IRF with multiexponential function and gives wavelength-dependent amplitudes of the fitted time components.23 The chirp of the white light continuum was about 800 fs in the given spectral range, but all the analyzed spectra were corrected for this chirp. Therefore, after correction the time origin was the same for all wavelengths used in the global analysis. The quality of the global fit was checked by examining the fits at different wavelengths and by the chisquared values for different numbers of exponential functions. The nanosecond flash photolysis setup was the same as previously described.17 A pump pulse of 5 ns time duration at 640 nm and energy density of 0.3 mJ/cm2 were used for the sample excitation. Because of the highly dispersive nature of the observed processes, the complete kinetics were obtained by combining the decays measured at several different time scales. The time-resolved spectra were constructed from fitted kinetic traces recorded in the spectral ranges 430−700 nm every 10 or 20 nm. Femtosecond (fs) emission transients were collected using a fluorescence upconversion technique. The system described before17 consists of a femtosecond optical parametric oscillator (Inspire Auto 100) pumped by a Ti:sapphire oscillator Mai Tai HP (Spectra Physics) and coupled to an upconversion setup. The pumping beam was adjusted at different wavelengths in the range 580−665 nm, and its polarization was set to a magic angle in respect to the fundamental beam. The typical energy of the pump pulse was 150 pJ (average beam power 18 mW), corresponding to energy density in pulse of 500 nJ/cm2, but control experiments with 3 and 12 times smaller energy were also done. The sample was placed in a holder connected to a pair of translation stages (MTS series, Thorlabs). They move the sample in the x−y plane perpendicular to the pump beam preventing it from photodegradation during the measurements. The IRF of the apparatus was 300−350 fs (fwhm). All the upconversion measurements were performed in a 200 ps time window.
To analyze the decays in upconversion and flash photolysis experiments, extending over a large time scale, a stretched exponential function (eq 1) was used to fit the experimental transients. In the case of upconversion studies the function was convoluted with the IRF. A stretched exponential function is given by the following formula A(t ) = A 0e−(t / τ)
β
(1)
This function contains two parameters: the characteristic time τ and the dispersion parameter β (0 < β < 1, lower β values mean more stretched decay, extending the decay time scale). The averaged rate constant k of the process described by function 1 can be calculated as8 ⎛ τ ⎛ 1 ⎞⎞−1 k = ⎜ Γ⎜ ⎟⎟ ⎝ β ⎝ β ⎠⎠
(2)
The steady-state fluorescence and UV−visible absorption spectra were measured using FluoroMax-4 (Jobin-Yvone) and JASCO V-670 or JASCO V-550, respectively. The photovoltaic characteristics of the cells were measured using a Sun 2000 class A Solar Simulator (Abet Technologies, Milford, CT, USA) equipped with an AM 1.5 G filter, with the light intensity adjusted at 100 mW/cm−2 using a silicon reference cell (ReRa Systems, Wijchen, The Netherlands). Current−voltage curves were recorded on a Keithley 2400 SourceMeter (Keithley, Cleveland, OH, USA). All experiments were done at 293 K. The electron injection rate constant kei was calculated (from upconversion and femtosecond transient absorption experiments) as the difference of the averaged constants obtained for TiO2 (kTi) and Al2O3 (reference, kAl) cells kei = k Ti − kAl
(3)
and the electron injection quantum yield was calculated as the ratio ϕei = kei /k Ti
(4)
Similarly, the dye regeneration rate constant kreg was calculated (from flash photolysis experiment) as the difference of the averaged constants obtained for TiO2 cells filled with electrolytes (kelectrol) and ACN (reference giving recombination rate, kACN = krecom) k reg = kelectrol − k recom
(5)
and the regeneration quantum yield was calculated as the ratio φreg =
k reg k reg + k recom
(6)
3. RESULTS AND DISCUSSION 3.1. Opaque Cell for Better Efficiencies. First, the solar cells with opaque titania nanoparticle layer optimized for high efficiencies were prepared, sensitized with HY103 or HY103 with CDCA, and filled with four different electrolytes: EL1, EL2, EL3, and EL4. Figure 1B shows the current−voltage characteristics of the cells studied, while Table 1A collects the parameters of the cells: short circuit current density (JSC), open circuit voltage (VOC), fill factor (FF), and global efficiency (η). As stated in the Introduction, for HY103 dye the conduction band edge of TiO2 has to be shifted toward more positive values to obtain reasonable photocurrents, which decreases 196
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one with significantly lower JSC (EL2), and we also add a commercial AN-50 electrolyte for comparison. 3.2. Steady-State Absorption/Emission and Flash Photolysis Studies. More transparent cells were prepared to investigate the coadsorbent and electrolyte composition effects by time-resolved laser spectroscopy techniques. In these cells the smaller nanoparticles (diameter ∼10 nm) were used; the maximum absorbance of the dyes on the metal oxide layer was kept to be not larger than 1; and the active area of the layer was relatively large, 1.5 cm2. Table 1B collects the values of the short circuit currents (JSC) and open circuit voltages (VOC) of the devices. A cell made of Al2O3 nanoparticles instead of TiO2 material was used as a reference system for which the internal deactivation of the dyes should be similar to that on TiO2, but no electron transfer process occurs due to the large bandgap of the Al2O3 semiconductor.26−28 Indeed, a negligible photocurrent for HY103+CDCA/Al2O3/AN-50 is observed. For the titania solar cells, the trend in JSC and VOC values is similar to that observed for opaque cells (Table 1A), but, as expected, the photocurrents are lower. JSC in transparent cells is 4−6 times lower than in optimized opaque devices, and VOC is also lower due to lower Fermi level of electrons in titania with a smaller number of photoinduced electrons. Among transparent cells the photocurrent is 5 times higher for EL3 than that for EL2 and AN-50 electrolytes (Table 1B). JSC for EL1 (containing TBP) is lower than that for EL2 (without TBP). The cells with and without coadsorbent give similar photocurrents, but VOC is lower for those without CDCA. All the further spectroscopic data were measured with these complete cells, and therefore the changes in the observed dynamics can be directly related to the differences in the measured photocurrents. The absorption (maximum intensity at 614 nm) and emission (maximum intensity at 658 nm) spectra of HY103 in DCM solution are shown in Figure S1A (in the Supporting Information). The Stokes shift between absorption and emission band maxima is relatively small (1100 cm−1), and it is not much changed with respect to that in acetonitrile (1400 cm−1),29 the solvent of much higher polarity. This means that the change in the dipole moment between S0 and S1 states of HY103 is not large. Figure 2A shows the visible absorption spectrum of the selected TiO2/AN-50 solar cells with and without CDCA (HY103+CDCA/TiO2/AN-50 and HY103/TiO2/AN-50 samples), while Figure S1B (Supporting Information) presents the same for TiO2/EL2 samples. The absorption intensity maximum for the samples with CDCA is at 640 ± 5 nm and should be assigned to the monomer absorbed on metal oxide particles. Interestingly, without coadsorbent an additional blueshifted band at around 600 nm appears, and it is due to dye aggregates. The classical exciton theory predicts a splitting of the excited state into higher and lower exciton band in the aggregates, so their absorption and fluorescence properties depend on the geometrical distribution of the monomer units in the aggregate.30 The absorption blue shift is characteristic for H-type of aggregates that form parallel stacking dyes, as expected for molecules absorbed close to each other on the nanoparticle. Moreover, in H-aggregate the transition between the ground to the lower exciton state is forbidden, so they show negligible emission.30 It can be noted that a shoulder at 600 nm is also present for the samples with CDCA (Figure 2A and Figure S1B, Supporting Information), which means that although the formation of H-aggregates is decreased the aggregates also exist, to some extent, in these samples.
Table 1. Values of Some Photovoltaic Parameters of the Studied Solar Cells: Short Circuit Current Density (JSC), Open Circuit Voltage (VOC), Fill Factor (FF), and Global Efficiency (η)a (A) cell
JSC [mA/cm2]
VOC [mV]
FF [%]
η [%]
HY103/EL1 HY103+CDAC/EL1 HY103/EL2 HY103+CDCA/EL2 HY103+CDCA/EL3 HY103+CDCA/EL4
0.87 1.03 1.75 2.74 9.18 10.20
504 532 466 454 463 405
63 65 67 57 64 58
0.28 0.36 0.55 0.71 2.73 2.40
(B) cell
JSC [mA/cm2]
VOC [mV]
HY103+CDCA/Al2O3/AN-50 HY103+CDCA/TiO2/EL1 HY103/TiO2/EL2 HY103+CDCA/TiO2/EL2 HY103/TiO2/AN-50 HY103+CDCA/TiO2/AN-50 HY103+CDCA/TiO2/EL3
